Reversing membrane wetting in membrane distillation: comparing dryout to backwashing with pressurized air

Abstract

The critical failure mode for membrane distillation (MD) desalination is wetting through the pores of the hydrophobic membrane, which allows the saline solution to leak through and contaminate the permeate. The standard practice for reversing membrane wetting is to dry out the membrane for several hours before resuming the desalination process. An alternative method for mitigating MD membrane wetting is examined in this study, wherein pressurized air is pushed through the membrane from the permeate side for several seconds, forcing trapped water out before it can evaporate. To compare the wetting reversal methods, the liquid entry pressure (LEP) was surpassed with saline water at varied salinity. Then, either a 24+ hour dryout, a 10 second pressurized air treatment, or both were applied, followed by remeasuring the LEP. Pressurized air backwashing restored the LEP to 75% of the original value for lower salinity feeds. The backwashing method is hypothesized to achieve this superior result because it removes saline solution from the membrane without separating water and salts by vaporization, whereas the dryout method causes seawater within the membrane to evaporate, leaving crystalline solutes trapped within the membrane. Such trapped particles may act as a path for rewetting, and also impair permeate flux and system energy efficiency. For all three methods, membranes tested with higher salinity water had lower LEP restoration irrespective of the restoration technique used. A method for testing LEP with more accuracy was also developed, using stepwise pressure increases. SEM images showed that the restoration methods did not alter the membranes themselves, although there remains a possibility that the air backwashing can cause superficial tears. Keywords: membrane distillation, wetting, dryout, air backwash, cleaning, crystallization